Along with the recent rapid spread of internet, an access system is required to be increased in capacity, advanced, and economized, and meanwhile, PON has been investigated as a means for realizing that. The PON is an optical communication system in which one optical line terminal (OLT) and a portion of a transmission path are shared by users, using an optical multiplexer/demultiplexer using an optical passive element, to contribute to economization.
Currently, in Japan, an economic optical communication system, GE-PON (Gigabit Ethernet (registered trademark) Passive Optical Network) in which a circuit capacity of 1 Gbps is shared by up to 32 users with time division multiplexing (TDM) is mainly introduced, whereby an FTTH (Fiber To The Home) service is provided at a realistic price.
In order to respond to the needs of a larger capacity, 10G-EPON with a total bandwidth of 10 Gbps class has been investigated as a next-generation optical access system, and international standardization has been completed in 2009. This is an optical communication system in which an increase in capacity is realized by increasing the bit rate of a transceiver, while using a transmission path portion, such as an optical fiber, which is the same as that of the GE-PON.
In future, although it is considered that an ultra-high definition video service, an ubiquitous service, and so on are required to have a large capacity of more than 10 G class, when the bit rate of the transceiver is merely increased from 10 G class to 40/100 G class, there is a problem that the practical application is difficult due to an increase of cost required for system upgrade.
As means for solving the above problem, there has been reported a wavelength tunable WDM/TDM-PON in which wavelength tunability is added to a transceiver in an OLT so that the transceiver in an OLT can be increased in a stepwise manner, according to a bandwidth requirement, and time division multiplexing (TDM) and wavelength division multiplexing (WDM) are combined effectively (for example, see Non-Patent Document 1).